Method for the detection of abnormalities of electric motors

ABSTRACT

In a method for error detection of a brushless electric motor, at least one first motor parameter is measured or determined, and a second, estimated motor parameter is estimated on the basis of the first motor parameter. The second, estimated motor parameter is compared to a second, measured or determined motor parameter. An error of the electric motor can be found out according to the comparison.

TECHNICAL FIELD

The present invention relates to a method for detecting abnormalities ofa brushless electric motor.

BACKGROUND OF THE INVENTION

Brushless electric motors or electronically commutated electric motorsgain significance at an increasing rate. They replace in particularelectric motors equipped with brushes in many technical applications.The advantages over motors equipped with brushes involve above all lowefforts in maintenance because there is no need for commutator brushesexposed to wear and there is principally a higher efficiency due to theomission of commutator losses caused by brush contact resistances. Inaddition, functions can be realized in conjunction with ‘intelligent’electronic commutation devices that are not possible with brush-fittedmotors or can be reached only with major additional mechanical efforts.This additional effort relates to the operation in the zone of weakfields or in the field weakening mode and the realization of a very lowwaviness of the drive torque.

As the functions of mechanical, inherently much safer and more reliablecomponents (in this case the commutator brushes of a commutator motor)are replaced by mechatronic assemblies in brushless electric motors,appropriate measures must safeguard the fail-safety. A comparativelylarge number of possible errors are caused due to the relatively highcomplexity of the commutation electronics.

Reliable error detection is necessary especially for safety-criticalapplications of electronically commutated electric motors.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to disclose a method permittingerror detection in brushless electric motors.

This object is achieved with the features of the independent claims.

Dependent claims are directed to preferred embodiments of the invention.

The method for error detection in a brushless electric motor comprises ameasurement or determination of at least one first motor parameteraccording to the invention. A second, estimated motor parameter isestimated on the basis of the at least one first motor parameter. Thesecond, estimated motor parameter is compared to a second, measured ordetermined motor parameter. An error of the electric motor is found outaccording to the comparison.

Consequently, the invention provides monitoring the electric motorincluding the commutation electronics in the sense of an overall test.There is no monitoring of partial components, such as controllermonitoring and/or monitoring of end stages. Instead, the system of theelectric motor is monitored in all.

Favorably, there is detection of a first class of errors which can causeundesirable motion of the motor, a second class of errors causingelectronic clamping of the rotor so that a rotation is no longerpossible or highly impaired, and a third class of errors preventing themotor from the development of a torque as a result.

The method is preferably implemented for the error detection ofelectronically commutated, three-phase, permanently excited synchronousmotors SM. Said motors consist of the main assemblies stator with astator with a stator winding and rotor and include a control unit, inparticular a transistor inverter TWR determining an appropriateenergization of the stator winding and adjusting it by way of powerdrivers.

According to the invention, the second, estimated motor parameter isestimated on the basis of a model.

It is arranged for by the invention that an estimated current producinga motor torque or a quantity derived therefrom is estimated as a second,estimated motor parameter and compared with a nominal current producinga motor torque or a quantity derived therefrom as a second motorparameter.

According to the invention, the estimated current producing a motortorque or the quantity derived therefrom is estimated on the basis of atleast one phase motor current, preferably three-phase motor currents,and the rotor position or phase position of the electric motor.

Thus, the nominal torque is indirectly predetermined by way of atorque-producing current q_nominal. The torque-producing currentq_nominal is directly proportional to the torque T in the stationarycondition, while saturation effects are ignored. The necessaryenergization pattern on the basis of a predetermined q_nominal value canbe determined by a stator-oriented or magnet-wheel-oriented currentcontrol.

According to the invention, the rotor position or phase positionrelative to the stator of the electric motor is determined in that therespective actual phase position variation of the rotor relative to thestator is measured. The absolute phase position can be determinedtherefrom.

The exact angular position or phase position of the rotor can bedetermined by an absolute position measurement. The absolute measuringsystem is mounted e.g. on a shaft on which the rotor is seated. Saidsystem indicates at any time the exact angular position of the rotorrelative to the stator. For example, so-called resolvers, such asinduction meters or rotatable transformers, or Hall sensors can beemployed as an absolute measuring system.

The torque applied to the rotor can be determined from the exact angularposition or phase position of the rotor relative to the stator, inparticular with the knowledge of the phase currents.

It is provided by the invention that the phase currents are estimated onthe basis of phase voltages taking into consideration inducedcountervoltages proportional to rotational speed.

According to the invention, the temperature of the electric motor and/orthe windings of the electric motor are measured and taken into accountwhen estimating the estimated currents.

According to the invention, the method and the device are employedespecially for brushless electric motors in the field of motor vehiclesfor use in steering systems such as steer-by-wire systems, or electricservo steering systems, or brake systems such as brake-by-wire systems.

The method of the invention permits preventing errors which can cause anundesirable motion of the actor of the steer-by-wire system orbrake-by-wire system (first class of errors), an electronic clamping ofthe rotor of the motor (second class of errors), or errors due to whichtorque cannot be built up (third class of errors).

The electric motor driving the actuator is preferably redundantlydesigned for safety-critical steer-by-wire systems or brake-by-wiresystems. When an error is detected, the function of the failing electricmotor is then ensured by the redundant systems.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a schematic diagram of the method of the invention.

FIG. 2 is a sectional view of the method of the invention with respectto the error decision.

FIG. 3 is a schematic view of a variant of the method of the inventionincluding a monitoring arrangement on the basis of phase voltages of theelectric motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The diagram shown in FIG. 1 depicts the invention method of errordetection in the example of an electronically commutated, three-phase,permanently energized synchronous motor SM 1. Said synchronous motor 1includes a transistor inverter TWR 2 determining an appropriateenergization of the stator winding and adjusting the motor phasecurrents i_(u) 7, i_(v) 8, i_(w) 9 by way of power drivers.

The specification of a nominal torque occurs indirectly by way of thespecification of a torque-producing current iq_nominal 3 and afield-weakening current id_nominal 4.

The whole of the motor 1 inclusive the transistor inverter 2 ismonitored according to the invention.

To this end, the requested nominal value of the torque-producing currentiq_nominal 6 and data about the motor phase currents i_(u) 10, i_(v) 11,i_(w) 12 are sent to a monitoring unit 5.

The monitoring unit 5, 17 is used to determine an estimated value forthe torque-producing current iq_th 14 on the basis of the motor phasecurrents i_(u) 10, i_(v) 11, i_(w) 12 and a determined mechanic rotorposition angle ε_(R) 13. This determination is preferably carried out bymeans of a model-based reproduction of the transistor inverter 2.

The estimated value indirectly indicates also the torque 16 that istheoretically applied to a motor shaft or drive shaft 15.

In consideration of the dynamics of the current control circuit, theestimated value iq_th is compared with the nominal value of thetorque-producing current iq_nominal by way of an error detection unit17. An alarm is issued at 18 when a significant discrepancy prevailsbetween iq_th and iq_nominal. To consider errors in the stator windingsand the current-measuring sensor system, it is checked in additionwhether the sum of currents of the star-connection operated motor iszero:i _(u) +i _(v) +i _(w)=0

Observer structures or parity models can be used in a classical way totake into account the dynamics of motor current control.

For defined cases of application, said dynamics is considered only byadding a response time before an alarm. This is illustrated in FIG. 2.

FIG. 2 shows a design of a structure of decision for detecting an error17. The difference between the two input quantities iq_nominal 6 andiq_th 14 is produced at 19. The amount of the difference is comparedwith a threshold value s at 20. Said threshold value can also be variedaccording to further quantities (adaptive threshold value). Higherthreshold values are fixed in block-commutated motors compared tosine-commutated motors. A timer 21 is started when a significantdiscrepancy between the amount of the difference and the threshold valueis detected in step 20. The timer 21 is decremented again until thevalue zero when the significant discrepancies disappear. There is anoutput at 22 when a predetermined count of the counter is exceeded.

This method allows a quickest possible error detection of ‘serious’errors. The exact location of the error and the cause of trouble is lesssignificant for these applications. Thus, the method of the invention isfirst of all especially suited for a quick error detection of the threeclasses of errors described hereinabove, with only their appearancebeing indicated. Any extensions, which additionally allow indications ofthe error location and the cause of trouble, are however feasible andcan be integrated into the method.

The actuator concerned, e.g. the motor 1 driving the drive shaft 15, isdisconnected in the way of a ‘collective’ error treatment (fail-silentbehavior) when an error is detected, and redundant systems areactivated.

A detailed error diagnosis will then take place within the limits of arepair or automatically after the deactivation under operatingconditions suitable herefor by means of corresponding algorithms andsequences.

It is the special advantage of the invention that all previouslymentioned classes of errors induced by malfunctions of the control unit2 (automatic motion, electric clamping, failure of the motor) can bedetected due to the principle of the overall test in conjunction withthe test of current sums, irrespective of where the cause of the troubleis originally localized, e.g. an erroneous transfer of nominal values, adefective rotor position sensor system, or a defective end stage foractuation of motor 1.

A brushless motor 1 can also be impaired by errors which, although theyrepresent a deviation from a normal function, either impair the torqueproduction of the motor 1 not at all or only minimally such as errorsthat cause a reduction of a boosting factor in the control unit 2. Aslong as the method does not signal an error, it is not necessary todeactivate the actuator 1. The output torque generally corresponds tothe preset nominal value. The control unit 2 adjusts the errors in thesecases. Thus, said errors are seized by the ‘robustness’ of the controlcircuit and do not require any countermeasures.

The dynamics of error detection lies in the magnitude of the timeconstant of the motor current control and, hence, amounts to normally<<10 ms, meaning it is quicker than methods that do without theevaluation of the motor current data and instead use purely mechanicalactuator parameters such as rotor acceleration, rotor speed, or rotorposition.

Further, the error detection method is of universal use inelectronically commutated motors with a position sensor equipment,irrespective of the implemented principle of the motor control and,consequently, is especially well suited both for a magnet-wheel orientedand a stator-oriented control.

In contrast to methods where only the sense of direction of the torqueis evaluated, the present method favorably allows detecting also errorsthat cause torque increase or decrease. It also allows detecting errorscausing waviness of the drive torque. This is of great significanceabove all in systems with a tactile interface to the user, e.g. electricservo steering systems or steer-by-wire systems with a manual torqueactuator.

In contrast to other feasible methods for the detection of motor errors,this invention uses the estimation of the motor torque to finallyfurnish the important hint whether and how the motor still performs itsproper function (torque generation), and whether it is possible torefrain from a deactivation recommended by error detection methods ofpossibly parallel operation. This arrangement increases the availabilityof the overall system.

FIG. 3 shows a modified method. Instead of the phase currents i_(u) 7,i_(v) 8, i_(w) 9 which are complicated to determine and adapted to beintroduced by the end stage 26 of the motor control unit 2, the currentscan be estimated on the basis of the corresponding voltages to neutralor phase voltages u_(u) 23, u_(v) 24, u_(w) 25 that are easy todetermine in consideration of the speed-proportional inducedcountervoltage, and thus made available to the algorithm.

Because uncertainties increase, e.g. due to a non-lineartemperature-responsive motor constant, temperature-responsive windingresistances and a high a-c component of the phase voltages, higherthreshold values are adjusted with the result of longer error detectiontimes. In addition, the winding and/or motor temperature θ 28 and therotor speed ω_(R) 29 are detected and evaluated. On the basis of a motormodel 30, motor phase currents i_(u, th) 31, i_(v, th) 32, i_(w, th) 33are estimated from the input quantities and sent as input quantities toa monitoring unit 34. Based on a model-based reproduction of the controlunit 27, the estimated value for the torque-producing current iq_the isdetermined therein, and the determined mechanical rotor position angleε_(R) 34 is used as another input quantity. The estimated value for thetorque-producing current iq_th is then sent to the error detection 36.

One advantage in this modified method—apart from the omission of currentmeasurement—is that short circuits in coil in the stator can be detectedin addition to errors in the control unit.

The model is not supplied with the nominal value of the torque-producingcurrent iq_nominal in the embodiments described hereinabove. Instead,the current is estimated on the basis of the phase currents (FIG. 1) orphase voltages (FIG. 3) what is favorable due to its high degree ofexpressiveness and possible interpretation.

Also, methods are feasible which make use of the correlation between therequested torque of a brushless motor and the current consumption of theend stage for error detection. A high current consumption of the endstage without the presence of a correspondingly high value foriq_nominal will then indicate an error. A high value for iq_nominalwithout a corresponding current consumption of the end stage must alsobe assessed as non-plausible and, hence, also indicates an error.

Due to greater uncertainties, the decision thresholds for errordetection are raised once more, with the result that insignificanterrors remain unnoticed. It is disadvantageous that a statement aboutthe sense of direction of the torque is impossible. Grave errors thatcause inverting of the sign of the nominal value remain unnoticed. Dueto its relative simplicity, the method, is provided for defined cases ofapplication as an additional error detection method in particularlysafety-relevant systems.

1. Method for error detection of a brushless electric motor, wherein atleast one first motor parameter is measured or determined, and a secondestimated motor parameter is estimated on the basis of the first motorparameter, wherein the second estimated motor parameter is compared to anominal current producing a motor torque, and an error of the electricmotor is found out according to the comparison, wherein an estimatedcurrent producing a motor torque and a quantity derived therefrom is thesecond estimated motor parameter and is estimated based on at least onephase motor current and a rotor position or a phase position of theelectric motor, and wherein an absolute rotor position or an absolutephase position relative to a stator of the electric motor is defined bymeasuring a respective actual phase position variation of the rotorrelative to the stator and determining the absolute phase positiontherefrom.
 2. Method as claimed in claim 1, wherein the second estimatedmotor parameter is estimated on the basis of a model.
 3. Method forerror detection of a brushless electric motor, wherein at least onefirst motor parameter is measured or determined, and a second estimatedmotor parameter is estimated on the basis of the first motor parameter,wherein the second estimated motor parameter is compared to a nominalcurrent producing a motor torque, and an error of the electric motor isfound out according to the comparison, wherein an estimated currentproducing a motor torque and a quantity derived therefrom is the secondestimated motor parameter and is estimated based on at least one phasemotor current and a rotor position or a phase position of the electricmotor, and wherein the at least one phase motor current is estimatedbased on phase voltages in consideration of induced countervoltagesproportional to rotational speed.
 4. Method for error detection of abrushless electric motor, wherein at least one first motor parameter ismeasured or determined, and a second estimated motor parameter isestimated on the basis of the first motor parameter, wherein the secondestimated motor parameter is compared to a nominal current producing amotor torque, and an error of the electric motor is found out accordingto the comparison, wherein an estimated current producing a motor torqueand a quantity derived therefrom is the second estimated motorparameter, and wherein a temperature of the electric motor or windingsof the electric motor or both is measured and also taken intoconsideration in estimating the estimated current producing the motortorque.
 5. Method as claimed in claim 1, wherein the error of theelectric motor is detected by comparing a requested torque of thebrushless electric motor and a current consumption of an end stage thatactuates the electric motor.
 6. Method a s claimed in claim 1, wherein,if an error is found out, an alarm is issued.
 7. Method a s claimed inclaim 1, wherein, if an error is found out, an error report is issuedand a redundant system is activated.